Fluorescent Probe Solubilization in the Headgroup and Core Regions of Micelles: Fluorescence Lifetime and Orientational Relaxation Measurements

Abstract

Experimental results demonstrate that the fluorescent probes 2-(N-hexadecylamino)-naphthalene-6-sulfonate (HANS) and 2-(N-decylamino)-naphthalene-6-sulfonate (DANS) are solubilized in two distinct regions, that is, the headgroup and core, within micelles of cetyltrimethylammoniumbromide (CTAB), tetradecyltrimethylammoniumbromide (TTAB), dodecyltrimethylammoniumbromide (DTAB), cetyltrimethylammoniumchloride (CTAC), and tetradecyltrimethylammoniumchloride (TTAC). The fluorescence lifetime decays for both chromophores are biexponential in all the different micelles. The population associated with the shorter lifetime (τ1 ≅ 4−5 ns) is located in the Stern layer, where reduction of the fluorescence lifetime occurs because of quenching induced by the bromide counterions. The second population of chromophores is located in the hydrocarbon core region of the micelle. In this environment the chromophores have a considerably longer lifetime (τ2 ≅ 19−20 ns) because there is no significant quenching by bromide counterions. Evidence of water penetration places them fairly close to the core−Stern layer interface. Time-dependent fluorescence anisotropy is analyzed in terms of these two populations. The measurements show that the orientational relaxation of the probes in the hydrocarbon core region is considerably slower than orientational relaxation in the Stern layer. When the lifetime measurements and the orientational relaxation measurements are combined, the partitioning of the chromophores in the core and headgroup regions of the micelles can be determined.